Two-dimensional tetragonal and hexagonal lattices of transition metal carbides MC (M = Ti, Zr, Hf): Observation of two nodal loops and strong light-harvesting ability

Abstract

Two-dimensional (2D) transition metal carbides (TMCs) have captured immense interest for their fascinating physical and chemical properties. Among them, bare carbide MXenes exhibit a metallic feature, greatly limiting the potential applications in optoelectronic devices. Herein, based on the particle swarm optimization and first-principles calculations, we predicted the existence of two single layer (SL) TMCs, namely, tetragonal β-MC and hexagonal γ-MC (M = Ti, Zr, Hf). The two structures are more stable than previously reported α-MC and possess semimetal and semiconductor properties, respectively. We further reveal that the β-MC monolayers have two nodal loops around the Fermi level. SL γ-TiC, ZrC, and HfC have a bandgap of 1.59, 1.24, and 1.57 eV, respectively, and display strong light-harvesting ability in the visible light range. Our research expands the 2D TMCs family beyond the extensively existing MXenes and greatly enriches the carbon chemistry.